My WASR seems to like the Romanian surplus mags the best, guess that since that's what it came with that's what Century used as reference when they opened it up. Hungarians are also a very good fit, and I've not found a mag that won't run 100% if it fits at all. Filing on surplus mags is common for best fit.
As for furniture, if you bought a new WASR without a trap door you should have the Beechwood furniture which is better than average. You'll have to spend a few bucks to buy better. Mine was a 2013 with the plywood stock & trapdoor and mismatched handguards. A few coats of linseed oil improved it but never looked great. But didn't matter cause I wound up with a quad rail and a folding CNC Warrior stock. So I'd recommend shooting it for a while and figure out how you want to config it before you invest in better wood.
IMHO Ironwood designs has the BEST furniture, which looks great on a milled receiver.
I have one of the backloading 'Made in Romania' drums, just a range toy but a lot of fun. I'd also recommend getting a few 20 round mags as well, they're handy for bench shooting or 'confined spaces'. The Hungarian Tankers are the gold standard for 20s, but there are also Magpul MOEs, Tapcos & Korean steel 20s. There's threads on how to cut down a surplus 30 to a 20 as well, I've done it with a few shop tools and my fabricating skills are minimal.
The recoil spring on my WASR got pretty weak after 1k rounds, didn't really notice it til I picked up a Draco pistol. So I replaced it with a Wolff spring, made a noticeable difference.
There's also a lot of talk about the bolt hanging up on the hammer of the G2 trigger group. I filed the back of mine down just a bit. Plenty of threads on that as well. I also did the 15 minute polish job with some Eagle One cloth after a few hundred rounds and it really smoothed out the action.